Microfluidic valve devices have been used in various biomedical applications. One known application is to implant micro check valves into an eye to treat glaucoma. Glaucoma is a well known ocular disease that affects millions of people. Persons afflicted with this disease require treatment for life. The disease causes visual field loss and if left untreated, may result in permanent loss of vision, and is a primary cause of blindness in the United States and elsewhere. The exact cause of glaucoma is not known, but it is characterized by pathological changes in the optic disc and nerve fiber of the retina. Studies suggest that development of the disease may be attributable to various factors including elevated intraocular pressure.
The intraocular pressure of a normal eye typically ranges from about 10 to about 21 mm Hg, e.g. about 15 mm Hg. Intraocular pressures of eyes of patients having glaucoma often exceed 21 mm, although glaucoma may be present when intraocular pressures are normal. Elevated intraocular pressures are believed to be responsible for slowly damaging the optic nerve which, in turn, can cause blind spots in the field of vision. Total blindness may occur if the entire optic nerve is destroyed.
It is known to implant devices for draining fluid from the eye in order to reduce intraocular pressure. One known implant device is known as a Molteno® implant. Earlier generation Molteno® implants were non-valved, free-flow implants having a scleral plate to promote formation of a functioning bleb and a tube that extends into the anterior chamber of the eye. The tube allows aqueous humor to flow from the anterior chamber to the plate where it is absorbed. However, these types of ocular implant devices are designed for continuous drainage and, therefore, may result in excessive drainage of fluid. Further, these types of implants do not provide sufficient drainage control. Thus, devices of this type may not be optimal for regulating intraocular pressure.
Another known implant is known as the Ahmed® valve, which is manufactured by New World Medical, Inc. in Rancho Cucamonga, Calif. This valve includes a restrictive element to reduce hypotony issues of certain Molteno® implants. The Ahmed® valve includes a silicone tube attached to a polypropylene body and plate. The valve mechanism includes two silicone elastomer membranes, and the valve is designed to open at a certain threshold pressure (about 8 mm Hg).
The Ahmed® valve, however, may be improved to enhanced regulation of intraocular pressure. Initially, implantation of the Ahmed® valve may be complicated due to the large size of the device. Additionally, implanting the Ahmed® valve requires use of sutures, which is not desirable. Further, the Ahmed® valve involves use of the Venturi effect to reduce flow rate, but does not provide for blockage or flow cut-off at higher pressures. In other words, the Ahmed® valve, like the Molteno® implant, does not allow for “band pass” functionality and is not able to prevent excessive drainage of fluid at high intraocular pressures, e.g., temporary elevated intraocular pressures caused by rubbing or pressing of the eye.
Referring to FIG. 1, known micro check valves, including those valves used to treat intraocular pressure and in other biomedical applications, such as free-flow Molteno® implants, are characterized by cracking pressure and/or reverse leakage. Cracking pressure is a minimum pressure that is required to open a valve for forward fluid flow. As shown in FIG. 1, in which the “x” axis represents fluid pressure and the “y” axis represents a corresponding flow rate, upon exceeding the cracking pressure, the pressure/fluid flow rate relationship is not ideal or linear. Instead, the relationship of known micro check valves is non-linear.
Further, known micro check valve devices, including the Ahmed® valve, are characterized by reverse leakage, which involves negative back flows of fluid and particles through the valve (which should be closed) and into the eye. This imperfect bi-directional valve behavior limits the practical use of known micro check valves as a flow control component in integrated microfluidics systems, particularly in miniature pressure/flow rate operations, and these behaviors exist regardless of whether micro check valves are fabricated by bulk micromachining by selectively etching a silicon substrate, or by surface-micromachining methods, which involves building structures on top of a substrate.
Thus, cracking pressure and reverse leakage continue to be technical issues with known micro check valves, and known micro check valves have not been able to achieve both zero cracking pressure and zero reverse leakage in a single device. Such micro check valve devices can be further improved in other ways, e.g., by providing additional flow controls, which would improve intraocular regulation and other biomedical applications involving microfluidics devices, and providing smaller devices that can be implanted more easily and without sutures (e.g., in the case of an ocular implant).
Therefore, it would be desirable to have implantable micro check valves improve upon cracking pressure and reverse leakage to provide a linear or ideal fluid pressure-flow rate relationship rather than non-linear relationships as shown in FIG. 1. Such devices would enhance various biomedical applications including lab on-a-chip, drug delivery, fluid regulation and other applications. Further, it would be desirable to have micro check valves that are easier to implant in a patient's eye and that are capable of regulating intraocular pressure more effectively and with enhanced fluid flow control, e.g., micro check valves that allow fluid flow within a certain range of pressures or that serve as “band pass” microflow regulators that are able to prevent excessive drainage during temporary elevated intraocular pressure caused by, e.g., rubbing, pressing or hitting the eye. It would also be desirable to have such an implantable micro check valves that can be implanted without sutures. It would also be desirable to be able to fabricate such micro check valve devices on a commercial scale using surface micromachining and MEMS technologies. Such capabilities would enhance various biomedical applications and treatment of glaucoma and other pressure-dependent physical conditions and diseases.